The present invention relates generally to steam precipitation for producing resin powder from liquid resin solution, and, more specifically, to a jet assembly for mixing the steam and resin solution to produce an atomized spray from which the resin powder is precipitated.
Steam precipitation is a common process of producing solid resin powder, such as polycarbonate, by introducing a liquid resin solution, such as polycarbonate in methylene chloride (MeCl.sub.2) solvent, into steam which subsequently vaporizes the solvent to isolate the solid resin or polycarbonate in powder form. The process typically injects resin solution containing 10% to 30% polycarbonate by weight into high speed steam in a component known as a jet to produce a spray. The evaporation of the solvent from the spray precipitates solid granular polycarbonate powder which is conventionally recovered in a downstream precipitation piping loop.
Steam usage in the precipitation process is a significant cost factor and the efficiency of steam use may be evaluated by calculating a steam-to-resin ratio (S/R ratio) which is defined as the ratio of steam mass flow rate to the mass flow rate of the resin itself. Lower values of the S/R ratio indicate better steam efficiency. The operating S/R ratio in current precipitation jets typically varies from about 1.6 to more than 2.2 depending on process conditions and resin grade. The minimum operating S/R ratio is limited by the onset of undesirable powder properties such as large powder particles or agglomerated chunks thereof, and/or plugging of the precipitation jet and downstream equipment. Other factors in evaluating precipitation jet performance include the ability to operate efficiently over a range of resin solution flow rates, and optimization of the resulting resin powder properties including bulk density, handling characteristics, and powder size for example.
In one conventional precipitation jet, a plurality of cylindrical barrels are circumferentially spaced apart and feed a common annular impingement cone which in turn communicates with a common tubular throat and diffuser extending downstream therefrom. Each of the barrels includes a convergent inlet for injecting the steam axially into the barrel and a separate inlet for the resin solution which is injected immediately downstream of the steam inlet.
In operation, the spray from the steam and resin solution inlets flows axially downstream through the individual barrels and then flows radially inwardly along the impingement cone and in turn to the common throat and is discharged from the jet into a conventional precipitation piping loop from the diffuser. Resin films form in the individual barrels, on the impingement cone, and around the downstream throat which allows for greater opportunity for film buildup or solidification resulting in degraded performance. As the resin flow rate increases, undesirable plugs of partially precipitated resin tend to form within the barrels themselves. These plugs are ejected as large chunks or grow out of the barrel to form larger plugs in the impingement cone and downstream tubing. Preferentially higher resin flow may exist in some barrels due to small dimension discrepancies which tend to plug some barrels before others and therefore limit resin flow rate and steam efficiency. The fixed geometry of the precipitation jets requires steam supply pressure changes to effect steam flow control. However, changes in supply pressure alter steam temperature and velocity to the jet which are important factors in atomization and precipitation of the resin solution which in turn effects complex performance behavior of the jet.